Summary of Work: The long-term goal of this project is to understand DNA replication fidelity by multiprotein replication complexes. This year, we examined the influence of replication accessory proteins on the fidelity of T4 DNA polymerase. Kinetic measurements showed that exonuclease-deficient T4 DNA polymerase, alone or with clamp loaders gp44/gp62 and polymerase clamp gp45, displays decreased binding affinity for incorrect as compared to correct dNTPs and a deceased kcat for misinsertion as compared to correct insertion. Kinetic constants were similar with and without accessory proteins, indicating that accessory proteins had little effect on misinsertion. However, the kcat for T?T mismatch extension was 5-fold higher in the presence of the clamp loader and clamp proteins. Thus, in the absence of proofreading, these accessory proteins may promote stable misincorporation. Error rate determinations show that, for some mispairs, the accuracy of exonuclease-deficient polymerase alone is similar to that in the presence of clamp loader, clamp and single-stranded DNA binding protein (gp32). However, exonuclease-deficient holoenzyme complex is actually less accurate than the polymerase alone for some base substitutions. Thus gp45 may promote extension of mismatches by tethering the polymerase to DNA, which may be relevant to replication past lesions. The rate for 1-ntd deletions was similar for the polymerase with or without its accessory proteins, implying that misalignment errors arise during processive replication. Finally, the T4 replication accessory proteins reduced by *10-fold the rate at which exonuclease-deficient T4 DNA polymerase generated deletions of larger numbers of nucleotides, indicating that these proteins influence replication fidelity for other than single base mutations. This study and earlier related work on this project are important for understanding the molecular genetic basis for the initiating events in diseases and the risk posed to individuals in the population by exposure to DNA damaging agents.